Pickup



May 21, 1946. B, F, MQNAMEE I 2,400,564

PICKUP Filed Feb. 26, 194; 2 Sheets-Sheet 2 INVENTOR Bernard f. AIM/mac BY I ATTORNEYS blades.

. the like applied to the seismic mass of I for example, the centrifugal force applied thereto I -Patented May 1 UNITED, STATES PATENT OFFICE PICKUP y Bernard F. McNamee, Altadena, Calif.-, assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application February 26, 1943, Serial No. 477,319

5 Claims. (Cl. 111- 351) Thi invention is concerned with pickups and the direction transverse to that at which a mepickup of the invention can be used as an accelerometer and is particularly useful for measuring vibrations of rotating members such as propeller Pickups to be installed on propeller blades and e the like should not modify the vibrations of the propeller to a serious extent and should have substantially the same sensitivity of all speeds of rotation of the propeller, at all temperatures at which the propeller might operate and for various radial distances-from the axis of rotation of the propeller. These characteristics are particularly necessary in measuring the vibrations of a wind tunnel propeller, which may have a radius as large as and when rotating a speed of 400 R. P. M. may produce a radial accelerationas high as 600 g. at its tip. Moreover, in a wind tunnel the temperature may be as high as 150 F. or as low as 10 F.,' depending upon the problem being investigated.

As the result of my investigations, I have developed a pickup having a resonant, frequency that is independent of the centrifugal force and the pickup,

when it is mounted on a propeller. In consequence, the pickup of my invention does not have to be calibrated for each speed at which the propeller is operated or for a change in the radial distance from the hub at which the pickup is mounted. i r v When the pickup of my invention is employed on a propeller or the like, it should be of light weight, which means that in such case magnetic damping is inapplicable. For such installations,

- I prefer a pickup that is oil damped, preferably particularly with pickups adapted-to be mounted on a body to determine vibration of the body in by-an oil that has a low temperature coefllcient of viscosity, 1. e. one in which the rate of chan e of viscosity with temperature is small.

For use on propellers and thelike the reaction of the pickup is maintained low by making the mass of the pickup very small, say as little as 2 .ounces andby making the pickup small, flat,

shallow, and compact so as to lessen wind resistance.

Another important feature "in the preferred form of my pickup is the use of a small resilient -member disposed in the case and capable of increasing and decreasing its volume in response to decreases and increases respectively of hydro static pressure exerted thereon by the dampin fluid. Thus, a small rubber bulb containing air or other gas may be enclosed in the pickup to prevent leakage of oil out of the pickup due to the temperature changes, etc. 1

In summary, my invention contemplates the.

combination in a pickup adapted to be mounted on a body to determine vibration of the body in a direction transverse to that at which a mechanical force is applied to the movable member of the pickup which comprises a base adapted to be secured to the body with its axis disposed in the direction in which said force isapplied, a spring mounted movable member secured to said base the springs being so proportioned and-connected.

that the said force acting on the movable member produces a compressional force per unit length in one spring that is equal and opposite to a tensional force per unit length produced in the other spring, and means for detecting movement of the member relative to the base.

Inthe pickup of my invention adapted to be mounted on a rotating body to determine vibration of the body in the direction of its axis of rotation and transverse to its radius of rotation, the combination involves a base adapted to be secured to the body with its axis in line with said radius, a spring-mounted movable member secured to the base and movable along a line perpendicular to the radius, the springs being secured to the movable member and to the base as described hereinbefore and extending from oppo site ends and on opposite sides of the member. The movement of the member may thus be parallel or transverse to the axis of rotation.

These and other features of my invention will be more thoroughly understood in the light of the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a diagram showing one of the pickups of my invention installed at the tip of a rotating propeller blade;

, Fig. 2 is a schematic diagram illustrating the manner in which the resonant frequency of the pickup is maintained constant irrespective of a change in centrifugal force applied to it;

sponse of .Fig. 3 is a plan view of a Preferred form of my pickup with the top section of the case removed;

Fig. 4 is a sectional view of the pickup taken on the lines 4-4 of Fig. 3;

Fig. 5'is a detail of the seismic mass of the pickup as viewed on the line 5-5 of Fig. 3;

Fig. 6 is a sectional view of one of the springs of the pickup taken on the line 66 of Fig. 4;

Fig. 7 is a wiring diagram showing how the pickup may be connected in a Wheatstone bridge;

Fig. 8 is a view partly in section of a. terminal stud in the base of the pickup;

Fig. 9 is a view of this stud taken at right angles to the view in Fig. 8; and

Fig. 10 is a pictorial view of the gas-filled rubber bulb which is enclosed in the case of the pickup.

' In Fig. 1, a vibration at the tip of a blade of a rotating propeller blade 3. The pickup is designed to detect vibrations in a direction perpendicular to the radius of th e blade and therefore perpendicular to the direction of the radial centrifugal force existing at the tip and indicated by an arrow 5. Changes in electrical voltages or currents produced in the pickup in response to these vibrations are transferred to a suitable measuring mechanism through leads 1 connected to terminal studs 33, 33 of the pickup, these leads being cemented to the propeller blade throughout their length up to the hub. Here they are connected through suitable slip rings 7' or other means to the measuring device.

In vibration pickups and the like, a movable element, sometimes called a seismic mass, is resiliently or otherwise movably suspended from the case or base of the pickup. The reaction of centrifugal force on the seismic mass and the resilient supporting means of a pickup mounted on a rotating body may have several undesired effects, especially if the centrifugal force becomes many times the force of gravity. These undesirable effects include changes in resonant frequency, sensitivity, and frequency response. I prevent centrifugal forces from modifying the resonant frequency, sensitivity and frequency remy vibration pickup in the manner illustrated by Fig. 2. Here a seismic mass 9 having a long side parallel. to the direction of the vibrations to be detected is supported at opposite ends of such length by leaf springs ll, l3 of equal length and strength. One of these springs extends radially outward and the other extends radially inward as referred to the center of rotation, thereby being parallel and antiparallel respectively to the centrifugal force at the point where the pickup is mounted on the rotating member. The ends of the springs not secured to the seismic mass are secured to a case l5 of the pickup. The outwardly and inwardly extending springs will hereinafter be referred to as the "outer" and "inner springs. Means I 8 are provided in the pickup to detect any movements of the seismic mass 9 relative to the case l5.

If, as a result of vibration, the seismic mass of Fig. 2 is displaced from its neutral position in a direction perpendicular to the centrifugal force acting on the mass, 8. compressional force will be produced in the outer spring and a tension or stretching force on the inner spring. The components of the compressional force in the direction of vibration will partially counteract the normal restoring force of the outer spring. The component of the tension force in the direction of vibration will add to the restoring force of the pickup is shown installed inner s ring. If the center of gravity-of the seismic mass 9 is halfway between the points where it is attached to the springs, the compressional force acting on the outer sprin will be equal in bined restoring force acting on the seismic mass and the centrifugal force will by the two springs be proportional to the displacement regardless of the amount of the centrifugal force. Consequently, the resonant frequency, the sensitivity. and thefrequency response of the pickup will be unaffected by any centrifugal force in a direction perpendicular to the line of vibration of the seismic mass relative to the case.

Generally speaking, changes in resonant frequency, sensitivity and frequency response with centrifuga1 force may be eliminated as long as the compression force in the outer spring per unit length thereof is equal to the tension force in the inner spring per unit length thereof. If the two springs are of unequal length, the position of the center of gravity of the seismic mass should be positioned at such point as to produce such equality of compression to tension forces per unit length in the respective springs. In this case the total restoring force acting on the mass per unit displacement thereof will be constant regardless of any such centrifugal force.

To measure the vibrations of a propeller blade, I prefer to use a pickup in the form of an accelerometer of the variable reluctance or variable impedance type. As shown in Figs. 3 and 4, my preferred pickup 00111311588 a base plate 11 provided with bores IQ for mounting on the propeller and bores through which screws 2| may be passed to secure a cover case 23 thereto. On the base plate ill, a c-shaped core made of soft iron laminations is mounted in a flat position. This core comprises a straight section 25, with the ends comprising two. L-sections 21, 21 rigidly secured to the base plate by screws, rivets, or force ilt pins.

The ends of the L-sections 21, 21,remote from the straight section 25, provide pole tips which form an air gap, really a dielectric oil-filled gap. The tips of the core are secured rigidlyto so that the magnetic 7 suspension is illustrated in Fig. 4.

the case to prevent changes in the air gap. Pickup coils 3i are mounted respectively on the long arms of the L-sections. The coils are connected in series (so that in effect they are one coil) to terminal studs 33, 33' extending through the base plate. As is customary, the coilsare connected fields they produce in the core are in the same direction.

--A seismic mass 35 is resiliently suspended in the gap between the pole tips by springs according to the general principles hereinbefore described in connectionwith Fig. 2. The mode of Thus, the mass is supported in the air gap by outer and inner springs i3 and II that extend parallel to the base. Both springs are rendered rigid (reenforced) at their mid-sections by bending their edges to produce channel-shaped cross-sections as shown in Fig. 6. Hence, the resilient action of the two springs is obtained from theunchanneled sections adjacent each end of the springs.

The leaf springs are equal in length and are attached to the seismic mass at points equally,

spaced from the center of gravity thereof. The

springs are parallel to each other and are adapted to be disposed parallel to the radius of rotation and at. right angles to the axis of rotation of the propeller. They extend in opposite directions on opposite sides of the mass.

As shown in Fig. 5, the seismic mass comprises a c-shaped frame 31 which is clamped and cemented to the springs and a flat soft iron vane or tab at and a flat copper vane-or tab M lying in a single plane and rigidly secured to the 6 frame. The iron and copper vanes form a single plate divided by a line 43' into a copperarea and an iron area and these areas are so proportioned twisting and loosening of the terminals. The terminals themselves comprise still copper wires which are flattened in a small section thereof to produce flat upset edges ii. To install the terminals, a short. Bakelite cylinder 63 is cemented] at the inside end of a countersunk bore 65 extending through the base plate H in a plane to the densities of iron and copper and the lengths of the springs II and it to position the center of gravity properly in accordance with the principles hereinbefore set forth.

The springs H a'ndlt are positioned onoDP site sides of the air gap so that the flux in the I gap intersects both the copper and iron vanes and the line 43 separating them is transverse, and preferably perpendicular, to the direction of vibration.

As the seismic mass vibrates, the areas of the copper vane and iron vane in the gap between the pole tips vary in opposite directions thereby cooperating to change the apparent electrical I impedance of the pickup coils II, as more fully explained in the co-pending patent application Serial No. 477,320, filed February 26; 1943, by Washburn and Hoskins. These changes in impedance may be'detected and reproduced in any suitable manner: 3

In brief, the copper and iron tabs together form an armature which is disposed in the fleld of a section (i. e., the C-shaped'core of soft iron lamterminals of a Wheatstone bridge. Arms of one branch comprise the pickup coils 3|, 3| conparallel to the face of the base plate. The copper wire is inserted in this Bakelite cylinder and a longer Bakelite cylinder 61 placed around the wire and in the countersunk bore. The upset edges of' this wire cut the-Bakelite cylinders as they are pressed together and the entire terminal assembly is held in place by rubber cement.

In'Fig. 7, I have shown a Wheatstonebridge arrangement for detecting the changes in impedance and therefore the vibration of the pickup.

Here an oscillator 45 is connected across input nected in series (and hence considered as one coil for purposes of illustration) with balancing resister 41. The other arm of this same branch comprises a dummy pickup 49 or other balancing circuit which has approximately the same impedance as thepickup and'also a balancing resistor ii. The other branch of the Wheatstone bridge comprises balancing impedances 53, 53',

58 of any suitable form. As the pickup vibrates, its impedance varies,'a, variable part 55 of the im- 'pedance being in series with a constant portion inations) which is of relatively low'reluctance per unit length and forms part of a magnetic circuit. The iron tab has relatively high paramagnetic susceptibility, low retentivity and relatively high electrical resistance and the copper tab is non-magnetic and of relatively low electrical resistance, 1. e., resistance to eddy currents.

The movement of the iron tab into the magnetic field at the air gap increases the inductance of the neighboring coils and at the same time electrical losses due'to eddy currents in the copper tab bring about an apparent increase in the electrical resistance in serieswith these coils. Consequently, the "effect .of the simultaneous movement of the'iron tab and the copper tab -are both in the samedirection and such as to affect the apparent impedance change of the coils; and this change is greater than it would be if only the iron tab or the copper tab were present.

changes, the volume of the rubber tube changes in a compensating manner. An organo-silicate oil is used for damping. It has a low temper- As shown in Figs. 8 and 9, special sealing means are provided to prevent leakage of oil between .the base plate, and the terminals and to prevent 51. Due to the vibration, changes occur in the relative impedance betweeni the two arms including the pickup and dummy pickup respectively thereby producing corresponding changes in voltage aor'oss the output terminals of the bridge. Such changes in voltage in the output terminals occurring in response to the vibration may be reproduced and detected in any convenient manner, one of which is more fully described in copending application of Raymond C. Oleson, Serial No. 477,321, filed February 26, 1943.

It should be observed that the springs which hold the seismic mass of the pickup and in which forces are produced due to the centrlfugalforce acting on the propeller are prevented from buckling by reinforcing their mid-sections.

It should also be noted that the reaction of the pickup on the vibrating body (say the propeller) is maintained low by (a) the low mass of the pickup (which may be as little as two ounces) and (b) by the shape of the'pickup, which is small, flat and compact with a shallow case. The pickup is preferably mounted on the rotatin member with its largest. dimensions in contact with the surface thereof. This reduces the wind resistance and also the induced vibrations due to turbulence of the air.

. ature coeflicient of viscosity and does not attack The terminal studs for making connection to the coil of the pickup project from the body of the pickup close to the base and extend parallel to the face of the pickup. This rmits the lead wires to be cemented to the surface of the propeller throughout their length,

It should be observed that, with some sacrifice of sensitivity, either the copper or the iron tab may be omitted. c

I claim': e

1. A pickup adapted to be mounted on a ing body for determining vibration in the direction parallel tothe axis .of rotation, said pickup comprising a base for attachment to the body, a core of a magnetic material, said core having an air-gap, an armature in the shape of a plate located within the air gap and adapted to move rotatthe armature at one in the plane or the plate relative to the core in said parallel direction, -a first spring secured to side 01' the core, a second spring secured to the armature at the other side of the core, said springs being fastened to the base at points which are in opposite directions from the armature.

2. A pickup adapted to be mounted on a rotat-' ins body for determining vibration in the direction parallel to the axis of rotation, said pickup comprising a base for attachment to the body, a core 01' magnetic material and a pair 0! coils mounted on said core for inclusion in a bridge circuit said core having an air-gap which extends in said parallel direction, an armature in the shape of a plate fitted into said air-gap and adapted to move relative to, the core in said parcomprising a base forv attachment to the bo y. a core or magnetic material having an air-gap which extends in said parallel directiom'a flat plate-like armature in the air-gap and adapted to vibrate relative to the core in saidparallel direction and a pair of resilient leaf sprinss attached to said armatureon op ite sides 01' the air-sap. said leaf springs extending from the armature in opposite directions, and means for attaching the springs to the base at points which are substantially equidistant from the armature.

4. A pickup according'to claim 8 in whicha cover is attached to the base so that the cover and base enclose the core, armature and springs, a damping tiuid within the cover, ands. resilient member within the cover capable of increasing allel direction, said armature being supported by two resilient members attached respectively at opposite sides of the air-gap, said members being fastened to the base at points which are on opposite sides of the air-sap along the radius of rotation.

3, A pickup adapted to be mounted on a rotating body for determiningvlbration in the direction parallel to the axis of rotation, said pickup and decreasln8 its volume in response .to contraction and expansion mpectively of the fluid.

5. A pickup according to claim 8 in which the armature. comprises two portions one or which is of magnetic material and the other or which is of an electrically conducting non-magnetic material, said portions being so ar-ranged that when one portion tends to move out other tends to move into'the air-gap.

g BERNARD 1". MCNAMEE.

of the air-sap, the 

